1. Field of the Invention
The present invention relates to shading coil and core designs of an electromagnetic pole. More specifically, the invention relates to shading coil and core designs of electromagnets used in electric brakes in hoists.
2. The Prior Art
Shading coils have been used in the past to influence the phase angle of the magnetic field of the encircled portion of a pole of an electromagnet. In electric brakes, where a brake plate is held against the force of a spring by an electromagnet excited by an alternating current, shading coils have been used to prevent the momentary total collapse of the magnetic force as the current is reversing. If the collapse of the magnetic field were not prevented, the brake parts would undergo an undesirable chatter at a frequency equal to twice the frequency of the alternating current.
The prior art shading coil has conventionally been pressed into slots formed across the face of the magnetic pole. In some cases. One leg of the loop of the shading coil would complete the circuit around the outside of the pole. While of simple construction, this design had the difficulty that the coil would loosen and eventually fall out as the result of brake cycling between on and off positions. Repair would require pressing a new coil into the slots in the pole face with special pressing and staking equipment. Such repairs may require shipping the hoist back to the plant for repair. Additionally, repair was only of a temporary nature, however, since the new coil would also eventually loosen and would require replacement. Each successive replacement would have a shorter lifetime since each successive replacement would cause the smoothing of the retention surfaces thereby weakening their holding ability.
Thus, it may be appreciated that a more reliable and consequently less expensive design for shading coils in hoist brake units is desirable. Such a design has been provided by the present invention.